Perhaps the most important dispute in the metaphysics of time is over the passage of time. There are two basic metaphysical theories of time in this dispute. There is the A-theory of time, according to which the common-sense distinction between the past, present and future reflects a real ontological distinction, and time is dynamic: what was future, is now present and will be past. Then there is the B-theory of time, according to which there is no ontological distinction between past, present and future. The fact that we draw this distinction in ordinary life is a reflection of our perspective on temporal reality, rather than a reflection of the nature of time itself. A corollary of denying that there is a distinction between past, present and future is that time is not dynamic in the way just described. The A-theory is also variously referred to as the tensed theory, or the dynamic theory of time. The B-theory is also referred to as the tenseless theory, or the static, or block universe theory of time.
The A-theory comes in various forms, which take differing positions on the ontological status granted to the past, present and future. According to some versions, events in the past, present and future are all real, but what distinguishes them is their possession of the property of pastness, presentness or futurity. A variant of this view is that events are less real the more distantly past or future they are. Others hold that only the past and present are real; the future has yet to come into existence. Still others, presentists, hold that only the present is real. Events in the past did exist, but exist no longer, and events in the future will exist, but do not yet exist. According to the B-theory, all events, no matter when they occur, are equally real. The temporal location of an event has no effect on its ontological status, just as the spatial location of an event has no effect on its ontological status, although this analogy is controversial.
Since the 1990s there has been much debate between the proponents of these ontological components of the A-theory and the B-theory: presentism and eternalism. Presentism has been in the ascendance, so the focus of debate in the philosophy of time has narrowed in on the question of whether it is sustainable philosophically and scientifically. For the opposing view, eternalism, the question has been whether it is sustainable in the light of our everyday experience of time.
The A-theory has a greater claim to being the theory that reflects the common-sense view about time. Consequently, the burden of proof is often thought to be on the B-theorist. If we are to give up the theory of time most closely aligned with common sense, it is argued, there must be overwhelming reasons for doing so. However, the A-theory is not without its problems. McTaggart put forward an argument that an objective passage of time would be incoherent, so any theory that requires one cannot be true. The A-theory also appears to be, prima facie, inconsistent with the special theory of relativity, a well-confirmed scientific theory.
Although the B-theory is less in line with common sense than the A-theory, it is more in line with scientific thinking about time. According to the special theory of relativity, time is but one dimension of a four-dimensional entity called spacetime. The B-theory sees time as very similar to space, so it naturally lends itself to this view. However, it faces the problem of reconciling itself with our ordinary experience of time.
Because the two theories about time are mutually exclusive, and are also thought to exhaust the possible range of metaphysical theories of time, arguments in favour of one theory often take the form of arguments against the other theory. If there is a good reason for thinking that the A-theory of time is false, then that is equally a good reason for thinking that the B-theory of time is true, and vice versa.